A polyurethane foam molded article having a high density surface layer (hereinafter referred to as a “skin”) has very good surface properties in applications other than those where it is covered by a skin made of other material, such as a mattress and cushion. Therefore, a flexible polyurethane foam and a semi-rigid polyurethane foam are often used in furniture, automobile interior components such as an arm rest, a steering wheel and a shift lever knob, shoe soles, and sporting goods. Rigid polyurethane foam has many applications such as an imitation lumber and a structural material.
Articles such as an automobile steering wheel and a shoe sole which require high wear resistance, for example, are usually made of a molded member covered by a skin on the surface thereof (or on the bottom in the case of shoe), with the inner portion (core) desirably having a lower density for better touch feeling.
Saddles of bicycles are also typically covered by a skin because a high strength is required of the surface with which the body of the rider makes contact and the portion where the saddle is mounted on the bicycle, with the core desirably having a lower density and softness for better ride comfort. In order to finish the painted surface with better appearance, the surface layer is required to have a high density.
In case of a structural material made of a rigid polyurethane foam, too, it is common to provide a skin in order to finish the painted surface of the molded article with better appearance, while it is required to make the core from a foamed material of low density in order to reduce the weight of the molded article.
A polyurethane foam molded article will have higher commercial value and find wider application, if a skin is formed at only a portion where it is required during use and the core is made of a foamed material having lower density and lower hardness in accordance to the required performance.
As a method for foaming a polyurethane, there is known a technology using a foaming machine in which carbon dioxide or a so-called low boiling point liquid such as fluorinated hydrocarbon is used as the blowing agent and a colorant component as a third component is metered to be provided by the foaming machine. Foaming machines specially designed for producing foamed polyurethane products by metering and mixing at least three components are also commercially available.
However, this equipment and these molding techniques presuppose that a molded article having uniform density, uniform hardness and uniform composition in any part of the molded article can be obtained. Therefore, the object of the foaming machine is to feed a raw material having a uniform composition including a blowing agent from start to finish during feeding the raw material.
Plastics, Vol. 24, No. 8, p. 118 discloses that a molded article having an skin layer, so-called a polyurethane integral skin foam can be molded in one time by using a liquid blowing agent having a low boiling point near room temperature such as a fluorinated hydrocarbon, based on the difference of foaming reactions caused by the temperature difference between an internal core part and a surface layer contacting with the mold at the molding and that this molding process is a known technology.
A method for producing a polyurethane foam molded article by using a blowing machine of polyurethane resin is described in Polyurethane World Congress '97, p. 185, wherein components required for foaming, such as an isocyanate, a polyol, a catalyst, a crosslinking agent and a low boiling point blowing agent such as fluorocarbons, are charged separately by metering pumps from respective tanks into the mixing head.
Urethane Technology, October/November 1994 discloses a method using water which is the most common as the blowing agent of the polyurethane resin. However, it is very difficult to form a sufficient skin on the polyurethane molded article by this method.
JP-A-5-59146 discloses a method for imparting a skin to a polyurethane resin molded article wherein the reactivity of the raw materials is increased by using a large amount of a urethane reaction catalyst or a cross-linking agent while the viscosities of raw materials for the polyurethane are increased.
JP-A-5-305629 discloses a method using a pyrolysis type blowing agent. JP-A-6-1820 discloses a method wherein water in the form of a hydrate salt is added to control the reaction between water as a blowing agent and an isocyanate.
In the technology disclosed by Plastics, Vol. 24, No. 8, p. 118, a ratio of the skin layer to the foamed core is almost constant, the skin cannot be formed at only parts on which the skin is required to be formed in the molded article, the density cannot be changed and the hardness of each part cannot be changed.
As in the production method disclosed in Polyurethane World Congress '97 p. 185, the molding technologies, the equipment and the like for metering and mixing at least three these components to produce the polyurethane foamed molding articles are designed such that a constant ratio of raw materials including a low boiling liquid blowing agent are continuously metered, mixed and fed. Therefore, a ratio of the skin to the foamed core is approximately constant. Thus, the skin cannot be formed at only parts on which the skin is required to be formed in the molded article, the density cannot be changed and the hardness of each part cannot be changed.
Urethane Technology, October/November 1994, JP-A-5-59146, JP-A-5-305629 and JP-A-6-1820 disclose methods for forming a skin on a polyurethane foam molded article. However, a molded article having a satisfactory skin layer is not obtained. Moreover, although it is possible to produce a stable polyurethane foam molded article in which the ratio of the skin to the foamed core is constant, it is impossible to produce a polyurethane foam molded article wherein the ratio of the skin to the foamed core is varied, and the skin or the high density is provided on only portions on which the skin or the high density is actually required for the molded article.
Thus, although the conventional methods are capable of molding the polyurethane molded article having uniform performance, such the conventional methods cannot produce the desired molded article because it is difficult to form the skin selectively on a surface of the molded article actually requiring the skin and to change density or hardness of each site by varying arbitrarily the proportions of the skin to the foamed core layer. There are no satisfactory methods.